How to Auto-Renew Let's Encrypt SSL Certificate?

We have a _few_ projects which use Let's Encrypt SSL Certificates and I intend to _continue_ using them.

## Tasks

+ [ ] Research, document & implement certificate renewal for:
+ [ ] hits.dwyl.io (Debian)
+ [ ] ademo.app #43 (Centos)

I feel like this should be a _standalone_ `.md` file similar to:
https://github.com/dwyl/learn-devops/blob/master/letsencrypt-wildcard-certificate.md

> Starting point: https://community.letsencrypt.org/search?q=auto

related to: https://github.com/dwyl/learn-devops/issues/43
required by: https://github.com/dwyl/learn-travis/issues/45